The invention is directed to an improved method and apparatus for controlling brake pressure in, and/or adapting brake pressure to, predetermined vehicle brake systems.
Known vehicle brake systems (German Offenlegungsschrift No. 32 37 659) have a tandem master cylinder with main pistons disposed in it, which in order to generate brake pressure are displaceable by means of a pedal via a tappet, for instance with the aid of a booster; two brake circuits and valves incorporated in them and each associated with one or two wheel brakes are provided, so as to be directed out of the normal position into a pressure reduction position if a braked wheel associated with them is in danger of locking, or skidding. This known vehicle brake system operates on the principle of direct pressure feeding from the pressure source. To attain an anti-skid or anti-locking (also known as ABS) function, the particular branch of the brake circuit located between the wheel brake and the valve is temporarily opened, causing pressure medium to flow out of the brake circuits so that the brake pressure drops in the desired manner. Once any danger of locking is past, then the pressure medium that has flowed out is replaced by feeding in pressure medium that has been kept ready, under pressure, by a pressure supply means. This known vehicle brake system requires a complicated electronic monitoring device, which prevents the valves from being directed into their pressure reduction position if the pressure supply should fail during an anti-locking situation.
In a further known hydraulic brake booster (German Offenlegungsschrift No. 28 25 087) equipped with an integrated anti-skid apparatus, the brake booster performs a dual function: not only boosting the braking force but also modulating pressure for anti-skid performance as well. For this pressure modulation, a particular valve circuit is used, together with a special brake booster structure in which the master cylinder of the brake booster is used for pressure modulation.
It is typical for these so-called integrated ABS systems that the integration includes the functional dual utilization of the brake booster both for boosting and for pressure modulation in the case of ABS functions. In such systems, the brake booster should have a short structural length, and in the event of pressure supply failure, a translation jump should occur, while if a brake circuit fails the brake pedal should not drop to the floor, and if at all possible there should be some indication of brake circuit failure and improper bleeding. Further objectives in such integrated systems are a perceptible, yet not overly strong, feedback at the brake pedal if an ABS function comes into play, as well as the inherent safety of the overall concept, which assures the functioning of both the brake booster and the brake circuits if individual components taking part in the function should fail, and does so without any additional monitoring circuit.
To meet these demands, hydraulic brake boosters with and without travel simulators for the brake pedal are possible; however, in order to be able to satisfy the above conditions, either the construction cost of the brake booster, with additional sensors and monitoring circuits, in systems operating with travel simulators, will be relatively high, or else major structural modifications of the brake booster must be made and it must be provided with additional pistons. A disadvantage shared by both types of systems is that such systems must necessarily be adapted to various vehicle types in terms of their design and basic function as well as in terms of their dimensions, so that many different master brake cylinder systems are necessary, with the attendant problems in terms of maintaining a varied inventory, not the least of which is the resultant expense thereof.
There is accordingly a need for a hydraulic brake booster concept which preferably uses an identical or perhaps conventional brake booster design and is in a position to be adapted in such a way to various external conditions of the vehicle or of a given braking event, taking ABS functions into account as well, as to enable the predetermining of arbitrary characteristic curves in the form of set-point courses of the brake pressure triggered at a given time, for instance via the pedal travel.